It is known to use in power capacitors a capacitor element which is comprised of hoops that are tightly wound from a strip of thin film dielectric material that has a metal layer applied to one or both sides. The capacitor element may have a circular-cylindrical shape.
It is also known to divide such a capacitor element into several sub-elements and to couple these elements in series. The sub-elements are disposed concentrically around each other, one externally of the other. The sub-elements are normally in an odd number. The sub-elements are wound so that they will all have the same capacitance. Capacitor elements of this described kind are described more specifically in, e.g., WO01/52284, WO 01/52285, WO 01/52286 and WO 01/54151. The capacitor element is wound extremely tightly so as to avoid air gaps between the turns of the winding. This tight winding of the turns means that binding turns that have already been applied will be compressed still further as winding continues. The outmost turn of the winding is not subjected to the force that is exerted inwardly from a plurality of outwardly lying turns. The capacitor element will therefore include very tightly wound film and a number of outermost turns in which although the film is tightly wound it does not fully have the same degree of tightness as the inwardly lying films.
When the capacitor element includes sub-elements, the outer portion of the outermost sub-element will thus be wound more loosely than the inner portion of this outermost sub-element and more loosely than the inwardly lying sub-elements. The winding pressure applied in respect of these latter sub-elements is relatively constant in practice.
It has been found that these less tightly wound outermost turns have poorer electrical properties than the remainder of the capacitor element. These outer turns of the winding will therefore set a limit on the possible dielectric strain. This has a negative influence on the economic use of the capacitor element, since the material consumption is inversely proportional to the square of the dielectric strain.
The object of the present invention is to provide a capacitor element of the kind in question in which the above problem is overcome.